Rollup 0.59.x removes important code from ReactDOM bundle

[gaearon]

Reproducing case:

1. Clone https://github.com/isidrok/react-bug
2. npm install
3. npm start
4. Open build.dev.js and scroll to diffProperties$1 function definition
5. Open node_modules/react-dom/cjs/react-dom.development.js for comparison and scroll to diffProperties$1 function definition

You’ll see that the original function definition contains a for loop like

function diffProperties$1(domElement, tag, lastRawProps, nextRawProps, rootContainerElement) {
  // ...
  for (propKey in lastProps) {
    if (nextProps.hasOwnProperty(propKey) || !lastProps.hasOwnProperty(propKey) || lastProps[propKey] == null) {
      continue;
    }
    if (propKey === STYLE) {
      // ...
    } else if (propKey === DANGEROUSLY_SET_INNER_HTML || propKey === CHILDREN) {
      // ...
    } else if (registrationNameModules.hasOwnProperty(propKey)) {
      // ...
    } else {
      // ...
    }
    // ...
}

However the diffProperties$1 definition in the Rollup’d bundle.dev.js is completely missing the (essential) logic under those conditions as well as conditions themselves, except for the last one. This produces code that behaves differently. Its for loop only contains the registrationNameModules.hasOwnProperty(propKey) check and not the preceding ones.

I verified that downgrading to 0.58.x fixes it.

[isidrok]

Didn't think about downgrading rollup since it was my first time trying out React 16.3... 😅

Seems it has something to do with treeshaking since you didn't change the version of rollup-plugin-replace

Thank you @gaearon !

[lukastaegert]

Thanks a lot for posting this issue + the repro. It took me a while to figure out but it seems this is actually a rather old issue that has come to the surface with the enhanced conditional tracking. Basically it appears that for..in statements will not count as assignments to their loop variable.

This is a minimal repro which is completely removed:

let x = false;

for (x in {key: true}) {
	if (x) console.log('removed');
}

I think it should not be too difficult to fix, will take a look soon.

[lukastaegert]

Fixed in Rollup v0.59.2, please check it out!

[isidrok]

Hi, the reproduction demo works fine with last rollup version, thanks!